Dark-adapted rods exert a tonic inhibitory influence upon cone-mediated flicker. This effect appears to be mediated in the distal retina. The proposed research involves an interdisciplinary study of this type of rod-cone interaction. Experiments with normal and color-blind human subjects have three goals: to fully describe all parameters which determine its magnitude; to compare and contrast it with two other types of rod-cone interaction; and to develop both psychophysical and ERG procedures which can quickly assess its extent in the clinic. Clinical research will use this effect to measure the extent of rod dark signals in patients with different types of night blindness, and attempt to distinguish between several forms of retinitis pigmentosa. Additionally, we wish to determine how precisely this effect can distinguish pathological from normal areas of the same retina. Intracellular recordings from distal retinal neurons should determine whether this type of rod-cone interaction can be explained in terms of horizontal cell feedback onto cones.